This invention relates to the field of electrical energy conversion circuits and to the improvements achievable therein using switching devices having both turn-on and turn-off capabilities.
Constant frequency alternating current energy is needed in modern aircraft, space vehicles and other transportation systems in what may be classed as moderate energy levels, levels falling in the range of one kilowatt to several hundred kilowatts of power, for example. Energy of this constant frequency nature is particularly needed for the operation of alternating current motors of the frequency responsive type such as induction and synchronous motors, and is also desirable in the optimization of transformers for maximum electrical capacity with minimum mass and space cost and in operating electronic equipment.
The need for electrical energy of this nature is, in fact, so intense as to have supported considerable prior art effort and a variety of approaches to its realization since the beginning of World War II, for example. These prior generation arrangements have included the use of a DC to AC rotating machine inverter, that is, a DC motor coupled to an alternating current generator; the use of constant speed separate prime movers or energy sources on the aircraft, i.e., rotating machine sources used solely for the purpose of generating electrical energy; the use of elaborate precision hydraulic transmissions by which a constant speed alternator can be driven from a variable speed engine; and to a limited degree, the use of variable frequency excitation in the rotating field of an alternator.
In more recent systems, systems accomplished since the advent of solid state electrical switching devices, the cycloconverter circuit has also been used to obtain this energy. In the cycloconverter an energy output waveform is fabricated from selected sample portions of higher frequency and often multi-phased input alternating current energy using a switching and possibly feedback controlled algorithm of sample generation. A related DC to AC form of this switching apparatus is commonly used in passenger bus lighting where fluorescent lamps are operated from a DC or battery source. A similar arrangement is also used in many of the battery energized portable fluorescent lantern devices employed by woodsmen and campers.
Although DC to AC inverters are related to the cycloconverter, the true cycloconverter may be thought of as an apparatus having both alternating current energy input and alternating current energy output and involving the use of input waveform sampling for fabricating or constructing the output waveform. In some cycloconverters, rectifier devices are employed in fabricating the alternating current output waveform. In yet another variation of the underlying concept, some such circuits are arranged to provide variable frequency output energy.
Previous cycloconverter arrangements have been somewhat hampered by the unavailability in the electronic switching device art of a moderate power level capacity electrical switching element that is capable of performing both turn-on and turn-off switching operations in response to low level control signals. As is well known in the electronic art most previously available switching devices such as silicon controlled rectifiers or thyristors have the characteristic of remaining in the on or conducting state, once triggered, until current flow is interrupted by some external means. Such characteristics are also common to previously used vacuum tube devices such as the thyratron, ignitron and grid controlled rectifier.
All of these solid state and vacuum tube switching devices therefore use an operating cycle wherein triggering is accomplished after some delay into the alternating current cycle and the resulting conduction interval is continued until interrupted at, for example, a zero voltage point of the alternating current sinusoid. In cycloconverter use this switching characteristic is less than optimum and often penalizes the cycloconverter operation, especially with respect to output waveform ripple and power factor of the load imposed on the source alternator or transformer supplying the cycloconverter energy.
As is disclosed subsequently herein, both the cycloconverter output waveform ripple and the source machine power factor characteristics can be improved upon through the use of switching or commutating elements which are controllable in both the on and off switching directions and especially through the use of a newly appearing class of Metal Oxide Semiconductor (MOS) switching elements of this type.
In airborne uses of the cycloconverter it is usually desirable to obtain an output waveform which is sinusoidal in nature and has a constant frequency of 400 Hertz or cycles per second and to source this energy from one or more alternators having an output frequency which ranges from twelve hundred to eighteen hundred Hertz or from sixteen hundred to thirty-five hundred Hertz, for examples, depending upon the aircraft and the engine designs involved. Generally in cycloconverter circuits it is desirable to maintain at least a three-to-one ratio between the lowest alternator output frequency and the cycloconverter output frequency. The twelve hundred to eighteen hundred cycles per second alternator output frequency range of the first instance above just meets this three-to-one ratio, for example, assuming a four hundred cycle per second cycloconverter output frequency.
It is also interesting to note that a cycloconverter may be arranged to provide either single Phase or multi-phased output energy while operating from either single-phase or multiple-phased input energy sources. The use of a multiple-phase input and three-phase output configuration in the cycloconverter is especially popular in view of the prevailing use of three-phase systems in airborne and ground energy supply systems. The well-known concept that three-phase distribution systems operate at optimum efficiency and three-phase energy convenience for motor and low ripple rectifier uses, for examples, also support this usage.
The prior patent art includes several examples of earlier inverter or cycloconverter circuit arrangements which are of general background interest with respect to the present invention. Included in these prior art patents is U.S. Pat. No. 4,567,552 issued to Syunichi Hirose et al which is concerned with a phase control device for a power converter having a feedback signal and an error signal control arrangement in the triggering circuitry of a controlled rectifier element.
Also included in this art is U.S. Pat. No. 4,648,022 issued to Colin Schauder which concerns an alternating current converter circuit employing bilateral switches that are arranged by groups of three bilateral switches connected respectively between each phase of the input alternating current source and one phase of the output alternating current wave conductors. The Schauder invention provides for a hidden DC link between the input and output conductors and a feedback controlled pulse width modulation algorithm which embodies a bang-bang technique for selecting the used sinusoid waveform portions. The Schauder patent also discloses the use of current sensing and the improvement of waveform quality through the use of an artificial thirty degree phase shifting arrangement that accomplishes reduction of the fifth and seventh harmonic content in the output waveform. The Schauder patent also contemplates the use of software in controlling commutating switch operation and envisions use of the invention in a variable frequency alternating current motor drive control. The Schauder patent also does not specify the employed turnoff switches, however, switch control by a gate signal appears to be desirable.
Also included in this prior art is U.S. Pat. No. 4,589,059 issued to Morihiko Tanino which concerns an arrangement for startup of a current-fed inverter having primary utilization as an induction heating power supply. The Tanino patent contemplates the use of artificial pulses for triggering thyristors until such time as the output current exceeds a latching current value in order to achieve a reliable and rapid startup of the inverter's operation.
While each of these prior art patents relates to an improved inverter operating arrangement, none of their disclosures envisions use of the input sinusoid wave sampling taught herein and in order to achieve improved output waveform ripple and improved energy source power factor. In addition, none of these prior art patents teaches the use of switching elements which can be controlled in both the on and off direction by way of a low level gate signal.